Heat treatment apparatus and heat treatment boat

ABSTRACT

A heat treatment boat houses a plurality of semiconductor wafers in a manner separated at intervals for heat-treating the wafers in a heat treatment furnace. The heat treatment boat includes a bottom plate, a first support rod erected on the outer peripheral edge of the bottom plate, a second support rod and a third support rod both erected on the bottom plate so as to make an central angle of 105° to 120° with the first support rod with respect to the center of the respective wafer supported by the rods, and a top plate provided opposed to the bottom plate for holding the rods. The stresses applied to the wafers housed in the heat treatment boat are distributed equivalently to three contacting points with the rods and become the minimum.

This is a continuation of application Ser. No. 08/679,612 filed on Jul.10, 1996, now abandoned, which is a division of application Ser. No.08/452,505, filed on May 30, 1995, now U.S. Pat. No. 5,586,880, issuedDec. 24, 1996, which is a continuation of application Ser. No.08/184,915 filed on Jan. 24, 1994, now abandoned.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates to a heat treatment apparatus forheat-treating circular-plate like articles to be treated such assemiconductor wafers or substrates and also relates to a heat treatmentboat.

B. Description of the Related Art

As one of the processes for manufacturing semiconductor wafers orsubstrates, heat treatment is used for forming oxidized films anddiffusing dopants at a high temperature. Horizontal heat treatmentfurnaces were mainly used in the past, but vertical heat treatmentfurnaces have recently come to be mostly used, because the latter takesin little air in the atmosphere.

A heat treatment apparatus using a vertical heat treatment furnace,i.e., a vertical heat treatment apparatus employs a vertically elongatedheat treatment boat (or a wafer boat) accommodating a lot of wafersvertically spaced from one another and loading them in and unloadingthem from the vertical heat treatment furnace. The heat treatment boatcomprises a circular top plate, a circular bottom plate disposedthereunder so as to be opposed thereto and four support rods made ofquartz or the like and provided between the top and bottom plates. Twoof the four support rods are arranged at such positions that theysupport the right and left portions of the front end part of each waferin view of the direction in which the wafer enters the heat treatmentboat. The support rods are provided on a heat insulating tube made of aheat insulating material.

Each support is formed with horizontal grooves having a width slightlylarger than the thickness of a wafer fitted in the corresponding grooveso as to support the corresponding outer peripheral edge of wafer. Thewafers are fitted in and taken out of horizontal grooves of the supportrods by a transfer arm in a space defined between the two front supportrods.

After a predetermined number of wafers have been mounted on the heattreatment boat, it is lifted by an elevator and introduced into the heattreatment furnace. Then, the wafers are loaded in the heat treatingfurnace, and predetermined heat treatment is performed.

In one of heat treatment processes, wafers are heated at such a hightemperature as 1200° C. for a long time in order to diffuse, to apredetermined depth, dopants (impurity ions) which are formed by ioninjection. When the mother material of the wafers is silicon, its yieldstress at this high temperature becomes such an extremely small value asabout 1/560 of its yield stress at the normal temperature (roomtemperature).

The diameter of wafers has become larger from 6 inches to 8 inches, andwafers having a diameter of 12 inches are now being developed. When,however, the wafers having such a large diameter are heat-treated at ahigh temperature close to the melting point of the mother material ofthe wafers, surface defects called slip are produced at the portionssupported by the support rods of the heat treatment boat. The slip isformed by minute faults which cannot be visually observed but can befound by a magnifying lens or under a microscope.

The causes of slip generation are:

(1) the internal stress due to the weight of a wafer itself and

(2) thermal distortional stress due to uneven distribution oftemperature on the wafer surface.

With respect to cause (1), since the wafer is supported at its circularouter peripheral edge by heat treatment boat and thus partiallysupported, a large internal stress is produced by the weight of thewafer itself at the supported portions. It is considered that slip isproduced when the internal stress exceeds a predetermined value.

With respect to cause (2), it is considered that slip is produced whenthermal distortional stress produced by the difference of thetemperature distribution between the central portion and the peripheralportions of the wafer as the temperature is raised exceeds apredetermined value.

The mechanism as to how is produced the internal stress due to theweight of the wafer, which stress is considered to be one of the causesof slip generation, will be explained more in detail in view of thestructure of the conventional heat treatment boat. Each wafer is notonly bent within a manufacturing tolerance but also due to uneventemperature distribution as the heating temperature is raised, and eachgroove of each support rod has a manufacturing error. The bending andthe manufacturing errors are combined to reduce number of the supportingpoints of the wafer from four to three as shown in FIG. 10. When thewafer is supported at three points, the loads on the three points becomeunbalanced as seen from the load distribution on the wafer around thesupport rods shown by Xs, and a large stress exceeding the limit of slipgeneration is exerted on one of the supported points Xs.

A method of reducing the internal stress due to the weight of a waferitself has been developed in which the supporting areas of the wafer arewidened by using the four support rods of the heat treatment boat madearcuated along the circular peripheral edge of each wafer.

However, it takes a lot of time and cost to machine members into supportrods having an arcuated cross section for ensuring a large supportingarea. In addition, each wafer is not always supported evenly by the foursupport rods over their whole supporting areas and is likely to receivean excessive load at its local portion. Thus, it is little guaranteedthat slip is effectively protected by this method.

When, therefore, wafers having a large diameter are heat treatedparticularly at a high temperature close to the melting point of themother material of the wafer, there occurs a problem that slip isgenerated. This makes it difficult to increase the diameter of thewafer.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a heat treatmentapparatus and a heat treatment boat for optimizing the internal stressdue to the weight of wafers themselves and reducing the generation ofslip.

In order to achieve the object, one aspect of the present inventionprovides a heat treatment boat for housing a plurality of plate-likearticles to be treated spaced apart from one another and locating thearticles in a heat treatment furnace for heat-treating the articles,comprising:

a bottom plate having an outer peripheral edge portion;

a first support rod erected on the outer peripheral edge portion of thebottom plate, for supporting the articles;

second and third support rods for supporting the articles, both erectedon the outer peripheral edge portion of the bottom plate on both sidesof the first support rod so as to make a central angle of 105° to 120°with the first support rod with respect to a center of the articlessupported by said rods, respectively; and

a top plate disposed over the bottom plate so as to be opposed thereto,for holding, together with the bottom plate, the first, second and thirdsupport rods.

Another aspect of the present invention is to provide a heat treatmentboat for housing a plurality of plate-like articles to be treated spacedapart from one another and locating the articles in a heat treatmentfurnace for heat-treating the articles, comprising:

a bottom plate having an outer peripheral edge;

a first support rod erected on the outer peripheral edge of the bottomplate;

a second support rod and a third support rod both erected on the outerperipheral edge of the bottom plate on both sides of the first supportrod circumferentially separated by 105° to 120° from the first supportrod, respectively; and

a top plate disposed over the bottom plate so as to be opposed thereto,for holding, together with the bottom plate, the first, second and thirdsupport rods.

A further aspect of the present invention is to provide a heat treatmentapparatus for heat-treating substrates to be treated, comprising:

a heat treatment furnace for housing and heat-treating said substrates;and

a boat for supporting the substrates and transporting the substratesinto the heat treatment furnace, and comprising a bottom plate, a topplate and three support rods which are arranged at three positionsseparated at equal intervals along an outer peripheral edge of each ofthe substrates housed in the boat.

A still further aspect of the present invention is to provide a heattreatment apparatus for heat-treating substrates to be treatedcomprising:

a unit for housing the substrates and heat-treating them; and

a unit for supporting the substrates and transporting them into thehousing and heat treating unit; and

the supporting and transporting unit having three-point supporting unitfor minimizing stresses applied to the substrates supported by thesupporting and transporting unit.

Since an article to be treated such as a circular semiconductor issupported by three support rods, particularly at their threecircumferentially substantially equidistantly separated supportingfaces, the article never fails to be supported by three support rodseven if the article is bent. It follows that each article to be treatedis always supported at three portions at which the load due to theweight of the article is substantially evenly distributed. Thus, a largeload is prevented from being applied to only one portion, and anexcessive stress is not concentrated thereto, whereby the generation ofslip on the article can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a heat treatment apparatus according toone embodiment of the present invention;

FIG. 2 is a perspective view of the main portion of the heat treatmentapparatus of FIG. 1;

FIG. 3 is a partial view of support rods of a heat treatment boat of theembodiment of the present invention;

FIG. 4 is a horizontal cross-sectional view of the support rods;

FIG. 5 shows an example how to form grooves in the support;

FIG. 6 shows another method of forming the grooves;

FIG. 7 shows a further method of forming the grooves;

FIG. 8 shows a central angle between the adjacent support rods;

FIG. 9 shows an example how to manufacture the support;

FIG. 10 shows stress distribution on a wafer supported by a conventionalfour-point support boat;

FIG. 11 shows stress distribution on a wafer supported by a three-pointsupport boat according to the present invention;

FIG. 12 is a graph showing the relationship between the distance betweenthe edge and a supporting point and the maximum shearing stress; and

FIG. 13 is a graph showing the relationship between the central anglebetween the adjacent supporting points and the maximum shearing stress.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be described withreference to the accompanying drawings.

FIG. 1 is a perspective view of a vertical heat treatment apparatus ofan embodiment according to the present invention.

As shown in FIG. 1, an I/O port 6 for transporting carriers 3accommodating a plurality of (twenty-five, for example) semiconductorwafers 1 is provided at the front portion of the casing 4 of a heattreatment apparatus 2. At the lower portion of the I/O port 6 isprovided a posture changing mechanism 8 for changing the posture of awafer 1 together with the carriers 3 by 90°. At the rear portion of theI/O port 6 is provided a carrier transfer mechanism 10 for receiving thecarriers 3 from the posture changing mechanism 8 and transporting thecarriers 3. The robot arms 12 of the carrier transfer mechanism 10 arelifted and lowered by a carrier elevator 14 and extends forward andshrinks rearward.

At the further rearward portion of the I/O port 6 is provided a transferstage 16 which can hold the carriers on two stories. Above the transferstage 16 is positioned a carrier stocker 20 having a carrier holder 22which can hold at most eight carriers 3.

At the rear portion of the carrier stocker 20 is placed a heat treatmentfurnace 24 which houses a heat treating container 22'. A rotary shutter26 is provided for opening and closing the lower opening of the heattreating furnace 24. Under the heat treatment furnace 24 is provided awafer boat 28 which can house at most 150 wafers and is moved up anddown by means of a boat elevator 30. Between the wafer boat 28 and thetransfer stage 16 is disposed a transfer apparatus 32 having a pluralityof (five, for example) hands for transporting the same number (five, forexample) of wafers at time.

In the heat treatment apparatus 2 of this embodiment as described above,the carriers 3 each housing a plurality of (twenty-five, for example)wafers 1 are mounted on the posture changing mechanism 8 in the I/O port6. The posture of each carriers 3 is changed by 90° and then transportedto the transfer stage 16 by the carrier transfer mechanism 10 or to thecarrier stocker 20. The wafers 1 in each carriers 3 are transferred tothe wafer boat 28 by means of the transfer device 32.

After a predetermined number of wafers 1 have been loaded, the waferboat 28 is lifted by the boat elevator 30. At the same time, the shutter26 is opened and the wafer boat 28 is inserted into the heat treatmentfurnace 24 through its lower opening. Thereafter, the shutter 26 isclosed and the wafers 1 are heat-treated by a heater in the furnace 24.After a predetermined heat treatment has been completed, the wafer boat28 is lowered and moved from the interior of the heat treating furnace24 to the starting position. In a predetermined time, the wafers 1 onthe wafer boat 28 are returned to the corresponding carriers 3 on thetransfer stage 16 by the transfer device 32.

The wafer boat 28 is formed with a lot of supporting grooves separatedat predetermined vertical pitches, in order to stack the wafers 1.

A heat treatment boat according to the present invention will bedescribed.

As shown in FIG. 2, the wafer boat 28 comprises a top plate 34, a bottomplate 36 disposed opposed under the top plate 34, and three support rods38, 40A and 40B made of SiC or polysilicone, for example, providedbetween the plates 34 and 36.

The supporting rods 38, 40A and 40B correspond to first, second andthird supporting rods, respectively. As will be described in detail, thesupporting rods 38, 40A and 40B are arranged at the portions whichdivide the circumference of the top plate 34 (and the bottom plate 36)into three substantially equal parts, i.e., at the portions which dividethe circumference of the wafer 1 into three substantially equal parts.

Between the second and third support rods 40A and 40B is defined anentrance space 37 for admitting hands of the transfer device 32 forloading wafers 1 on and unloading the same from the wafer boat 28 (FIG.2). As shown in FIG. 4, the first support rod 38 is positioned on ahorizontal central line L of the wafer 1 along which the wafer 1 moves,and the second and third support rods 40A and 40B are arrangedsymmetrically with respect to the central line L at positions closer tothe I/O port 6 than the first support rod 38.

As shown in FIG. 4 again, the first support rod 38 is a rectangular posthaving a rectangular cross section, for example, and the second andthird support rods 40A and 40B are arcuated posts each having a crosssection of a thick halved tube. The arcuated inner surfaces of thesecond and third support rods 40A and 40B are directed slightly deviatedfrom the center P of the wafer 1 toward the first support rod 38.

As shown in FIG. 3, the support rod 38 is formed with a lot ofhorizontal grooves 42, and the support rods 40A and 40B are formed witha lot of horizontal grooves 44 so that the grooves 38, 40A and 40Breceive and support the corresponding wafers 1. The lower faces or thesupporting faces of the grooves 42 and 44 for each wafer 1 are formed atthe same level so that they support the corresponding wafer 1horizontally.

Referring to FIG. 4, the groove 42 of the first support rod 38 has awidth of 15 mm and a depth of 10 mm when viewed from the side of thewafer 1. Each of the second and third support rods 40A and 40B comprisesa semi-tubular member having an outer diameter of 23 mm and a thicknessof 8 mm. Each of the grooves 42 and 44 of the second and third supportrods 40A and 40B is formed so that the inner wall of the groove isdisposed slightly outside of that locus of the outermost edge of thecorresponding wafer 1 which is drawn as the wafer 1 is loaded andunloaded. The height of each of the grooves 42 and 44 is about 2.5 mm soas to allow the wafer 1 is moved upward or downward when the wafer 1 isloaded and unloaded.

The positional relationship between the wafer 1 and the supporting facesof the grooves 42 and 44 will be described.

It is preferred that a line connecting the center P of the wafer 1 witha portion of the supporting face of each of the grooves 44 of the secondand third supporting rods 40A and 40B make an angle θ of 105° to 120°with a line connecting the center P with the center of the correspondsupporting face of the groove 42 of the first support rod 38 (the anglebeing hereinafter referred to as the "central angle"). Let it be assumedthat the outer edge of the supporting face of a groove 44 of the secondsupport rod 40 and the outer peripheral edge of the wafer 1 coincide ata point Q. In this embodiment, the angle defined between the line PQ andthe line connecting the center of the wafer 1 with the center of thesupporting face of the corresponding groove 42 of the first support rod38 is taken as the angle θ and set to 105° to 120°.

FIG. 13 shows a graph showing the relationship between the central angleθ and the maximum shearing stress ratio, in which the maximum shearingratio is 100% at θ=98°. As understood from the graph, the maximumshearing stress ratio is less than 80% in the range of θ=105° to 120°,and the ratio is the minimum at θ=114.5°.

The more inner portion of the wafer 1 is supported, the less is thestress applied to the wafer 1. In order to do so, the outer peripheraledge of the wafer 1 is separated from the supporting faces of thecorresponding grooves 42 and 44 by making the inner portions of thesupporting faces of the grooves lower than the outer portions thereof.

FIG. 12 is a graph showing the relationship between the maximum shearingstress ratio and a distance between the contacting point of the waferwith the supporting face of the groove and the outer peripheral edge ofthe wafer, in which the maximum shearing stress ratio is 100% at adistance of the contacting point of 5 mm. It is understood that themaximum shearing stress ratio is reduced to about 47% at the distance ofthe contact point of 15 mm.

FIGS. 5 to 7 show how to form and shape the grooves.

In the embodiments of FIGS. 5 and 6, circular holes 50 and oval holes 52are formed in the support rods 38, 40A and 40B, respectively, so as toextend in a direction crossing the direction of the thickness of thesupport rods 38, 40A and 40B, and slits are formed so as to extend fromthe inner edge of the support rods to the holes 50 and 52, therebyforming notched grooves 42 and 44 with which the outer peripheral edgesof the wafers 1 are not in contact. The opening edges of the supportingfaces of the grooves 42 and 44 are chamfered into a round form.

In the embodiment of FIG. 7, horizontal groove portions are formed inthe support rods 38, 40A and 40B first, and then groove portionsinclined upward toward inner surface of the support rods are formed bymeans of an inclined cutter 55 so as to form completed grooves 42 and 44as shown in the right support rod. These grooves are formed in thetemporary baking step prior to the regular baking step.

As shown in FIG. 2, the wafer boat 28 having the structure as describedabove is removably mounted on the heat insulating tube 33 provided onthe lower end with a flange. The heat insulting tube 33 is disposed onthe boat elevator 30, and the vertical heat treatment furnace 24 isplaced over the wafer boat 28. To the heat treating container orreaction tube 22' in the vertical heat treatment furnace 24 areconnected a gas supplying tube 23 for supplying a predetermined gas tothe reaction tube 22' and a gas exhausting tube 25 for exhausting thegas from the reaction tube 22'.

The operation of the embodied heat treatment apparatus will bedescribed.

Untreated wafers 1 are placed in the wafer boat 28 from the entrancespace 37 defined between the second and third support rods 40A and 40Bby means of the hands 32 of the transfer device or the transfer arms 28,and then inserted into the respective grooves 42 and 44. The wafers 1are transferred to the wafer boat 28 by lowering the hands 32 slightlywith respect to the wafer boat 28. In this way, the center of theorientation flat of each wafer 1, for example, is supported by thesupporting face of the corresponding groove 42 of the first support rod38, and both side edge portions of the wafer 1 with respect to itsentering direction are supported by the supporting faces of thecorresponding grooves 44 of the second and third support rods 40A and40B. As already explained, the wafer 1 is supported at slightly insideportions of its outer peripheral edge.

Loading and unloading of wafers 1 are carried out from the top stage tothe lowest stage of the wafer boat 28 in turn. After 150 wafers, forexample, have been mounted on the wafer boat 28, the boat elevator 30 islifted and the wafers 1 are placed in the vertical heat treatmentfurnace 24. For example, in order to perform heat treatment at about1200° C., the interior of the vertical heat treatment furnace 24 ispreviously heated to 800° C. After the wafers 1 have been loaded, theinterior of the furnace 24 is heated to 1200° C. to performpredetermined heat treatment. Thereafter, the boat elevator 30 islowered to unload the wafers 1 from the furnace 24. Then, the wafers 1are taken out of the wafer boat 28 in the reverse steps to the waferloading steps on the wafer boat 28.

Since the three supporting faces of the wafer boat 28 for each wafer 1are arranged at substantially equal intervals on the outer peripheraledge of the wafer 1 in this embodiment, the wafer 1 is supported well bythe three supporting faces, i.e., three-point support of the wafer 1 isattained effectively even if the wafer 1 is bent and/or there aremanufacturing errors in the levels of the supporting faces of thegrooves 42 and 44. As compared with the four-point support type waferboat used in the conventional vertical furnace as shown in FIG. 10, thethree-point support type wafer boat according to the present inventioncarries a large load on each supporting surface, if the four-pointsupport is perfectly attained in the conventional case. However, thefour-point support by the conventional wafer boat is likely to beunbalanced greatly so that an excessive load is applied to a specialsupporting face so as to generate a large stress in the wafer at thissupporting face. With the wafer boat 28 according to the presentinvention, such unbalanced localized stress is not produced in the wafer1, thereby reducing generation of slip in the wafer 1. The melting pointof silicon is 1,410° C. Thus, when silicon wafers are heat-treated atabout 1,000° C., the three-point support structure of the wafer boataccording to the present invention is very effective and useful.

The second and third supporting rods 40A and 40B are halved tubularmembers. Thus, the supporting faces of the grooves of the supportingrods 40A and 40B can be disposed inside of the wafer 1 by making thecentral angle (the opening angle between the first support rod 38 andthe second or third support rod 40A or 40B) θ large, thereby reducingstresses applied to the wafer 1. When, on the contrary, groovesextending along the entering loci of the wafers are formed in thesupport rods having a rectangular or circular cross section, it is adisadvantage that very little thickness is left in the supporting rods.Alternatively, it can be considered that necessary thickness is given tothe supporting rods by disposing them more outside of the wafers.However, such an arrangement is not preferable because it makes theouter diameter of the heat treatment boat large.

The relationship between the opening angle (central angle) between thefirst support rod 38 and the second or third support rod 40A or 40B wasstudied in the following way.

As shown in FIG. 8, a rectangular chips 60, 62 and 64 corresponding tothe first, second and third support rods 38, 40A and 40B were arrangedso that a line connecting the center of the wafer 1 with the center ofthe rectangular chip 60 and a line connecting the center of the wafer 1with center of the chip 62 or 64 made an opening angle (central angle)α. The wafers 1 disposed on these supporting chips 60, 62 and 64 wereheat-treated in the same way as in the above-mentioned embodiment, andthe surfaces of the wafers 1 were observed.

The opening angles (central angles) were 95°, 100°, 105° and 110°. Slipis reduced slightly at 105° and reduced remarkably at 110°. Further, noslip generation was observed in some samples. Therefore, it is necessaryto arrange supporting parts of the supporting faces of the first tothird supporting rods 30, 40A and 40B at substantially equal intervalsalong the outer peripheral edge of the wafer 1. More concretely, theopening angle α, i.e., the central angle θ between the supporting partof a supporting face of the first support rod 38 and the supporting partof the corresponding face of the second or third support rod 40A or 40Bwith respect to the center of the wafer 1 is preferably 105° to 120°.

The structure of the support rods is not limited to that of theabove-mentioned embodiment. As shown in FIG. 9, an oval tubular member68 may be divided into four sections 72, 74, 76 and 78 and thesesections may be used as the second and third support rods. In this case,as representatively shown in the section 72, a groove 70 which allowsthe outer peripheral edge of wafer 1 to be inserted thereinto is formedin each section. The structure permits the supporting faces of thegrooves 70 in the second and third support rods to support portions ofthe wafer 1 inside of its outer peripheral edge in a state in whichample thickness is left in the second and third support rods.

The present invention is not limited by the aforementioned embodimentsbut is naturally applicable to various modifications within the scope ofthe present invention.

What is claimed is:
 1. A heat treatment boat for supportingheat-treating substrates and transporting said substrates into a heattreatment furnace comprising:treatment support rods; and a means forsupporting three supporting rods at predetermined intervals from oneanother along an outer peripheral edge of each of said substratessupported by the support rods, said three support rods including a firstsupport rod, and a second and a third support rod provided on both sidesof said first support rod, each of said second and third support rodsbeing a substantially halved hollow tube having an arcuated crosssection defined by curved outer and inner surfaces, each of said first,second and third support rods having notched grooves, and each formedwith a lower face on which a corresponding one of said substrates sits.2. A heat treatment boat according to claim 1, wherein said firstsupport rod is disposed in front of said substrates with respect to anentering direction in which said substrates enter the boat, said secondand third support rods are provided on both sides of said first supportrod, and said second and third support rods define therebetween anentrance space through which said substrates enter the boat.
 3. A heattreatment boat according to claim 1, wherein said supporting meansincludes a bottom plate, and said second and third support rods areerected on an outer peripheral edge portion of said bottom plate on bothsides of said first support rod so as to make a central angle of 105° to120° with said first support rod with respect to a center of saidsubstrates supported by the boat, respectively.
 4. A heat treatment boataccording to claim 3, wherein each of said lower faces is inclineddownward radially outward.
 5. A heat treatment boat according to claim4, wherein each of said notched grooves includes a round hole and aslit.
 6. A heat treatment boat according to claim 4, wherein an edge ofsaid lower face of each of said notched grooves is chamfered.
 7. A heattreatment boat according to claim 1, wherein said support rods andsupporting means are made of SiC or polysilicone.
 8. A heat treatmentboat according to claim 1, wherein said support rods are disposed onsaid outer peripheral edge of each of said substrates at such positionsthat stresses applied to said each of said substrates become theminimum.
 9. A heat treatment boat according to claim 1, wherein saidsecond and third support rods are arranged so as to make a central angleof substantially 114.5° with said first support rod with respect to acenter of said substrate supported by the support rods.
 10. A heattreatment boat for supporting heat-treating substrates comprising:threesupport rods for minimizing stresses applied to said substratessupported by the support rods; and a plate for supporting three supportrods so that they are arranged at three positions separated at intervalsalong said outer peripheral edge of each of said substrates supported bythe supporting rods, two of said support rods erected on said plate soas to make a central angle of 105° to 120° with the remaining supportrod with respect to a center of said substrates supported by supportingrods, each of said two of said three support rods being a substantiallyhalved hollow tube having an arcuated cross section defined by curvedouter and inner surfaces, each of said three support rods having notchedgrooves, each formed with a lower face having a tip portion forsupporting a corresponding one of said substrates, and said lower facesof said three support rods for supporting the same substrate beingplaced on the same horizontal plane.
 11. A heat treatment boat forsupporting substrates, comprising:a bottom plate; and three support rodsarranged at three positions separated at predetermined intervals alongan outer peripheral edge of each of said substrates supported by thesupporting rods, said three support rods including a first support rod,and a second and a third support rod provided on both sides of saidfirst support rod, each of said first, second and third support rodshaving notched grooves each formed with a lower face having a portionfor supporting corresponding one of said substrates, said lower faces ofsaid first, second and third support rods for supporting the samesubstrate being formed on the same plane, each of said lower faces beinginclined downward radially outward, each of said second and thirdsupport rods being a substantially halved hollow tube having an arcuatedcross section defined by curved outer and inner surfaces.
 12. A heattreatment boat for supporting heat treating substrates, comprising:threesupport rods; and a means for fixing three support rods so that they aredisposed at predetermined intervals from one another along an outerperipheral edge of each of said substrates supported by the supportingrods, said three support rods including a first support rod, and asecond and a third support rod provided on both sides of said firstsupport rod, each of said second and third support rods being a hollowtube divided lengthwise having a curved cross section defined by curvedouter and inner surfaces, each of said first, second and third supportrods having notched grooves, and each formed with a lower face on whicha corresponding of one said substrate sits.